In optical data transmission, data may be transmitted via an optical waveguide by means of an optical signal having a specific wavelength, preferably called carrier wavelength. The optical signal may be modulated for indicating transmitted data, wherein the modulation may be carried out as amplitude- and/or phase-modulation in dependence on the data that is to be transmitted. A prominent phase-modulation method is for example Quadrature Phase-Shift Keying (QPSK). Another prominent modulation method employing amplitude- and phase modulation is for example 16-Quadrature Amplitude Modulation (16-QAM).
In order to increase the data rate, not only one optical data signal of the specific wavelength may be transmitted, but more than one optical data signal may be transmitted in a so called multiplexing technique.
In the multiplexing technique of Wavelength Division Multiplexing (WDM), multiple optical data signals with respective wavelengths different from each other are transmitted via the same waveguide. Each signal is modulated individually in dependence on the data to be transmitted via the respective signal.
Another multiplexing technique is that of polarization division multiplexing (PDM). In PDM, two optical signals, which have a same wavelength but respective polarization states that are orthogonal to each other, are transmitted via the same waveguide. Also in PDM, each signal is modulated individually in dependence on the data to be transmitted via the respective signal. The technique of PDM may be combined with that of WDM, in order to further increase the data rate.
An optical signal of a single wavelength can be described as an electromagnetic wave, that possess the single wavelength, and wherein the electrical field and the magnetic field are orthogonal to each other.
An optical signal with a specific wavelength propagating through an optical Single Mode Fiber (SMF) is often described as can be described as a Transverse Electro Magnetic (TEM) Mode, wherein the electromagnetic wave has an electrical field and a magnetic field that have no field component in the direction of propagation, and wherein the so called mode pattern describes the distribution of the electrical field in space within the plane that is perpendicular to the direction of propagation. Within an SMF, only a propagation of an optical signal with a fundamental mode is possible.
A so called optical Multi Mode Fiber (MMF) is a fiber, in which not only one optical signal of the specific wavelength may propagate as a TEM Mode as the fundamental mode, but multiple optical signals may propagate within the fiber as different TEM modes of the same specific wavelength. The different TEM modes differ in their respective mode patterns of their respective electrical fields.
A mode can be uniquely identified by its azimuthal periodicity and by the number of zero crossings in its radial function. Thus, a mode and its respective mode pattern are classified by a so called azimuthal order and a so called radial order. This leads to a unique pattern consisting of zones with positive and negative sign for the complex electrical field, equivalent to phase levels of 0 and pi. The azimuthal order with index I indicates the number of periods that a mode pattern has, when passing along a circular orbit around the origin of the mode pattern. The radial order with index m indicates the number of extrema that a mode pattern has along a radius going through the origin of the mode pattern. For modes of an azimuthal order of I=0, the radial order m indicates a number of 2 m+1 extrema. For modes of an azimuthal order of I>0, the radial order m indicates a number of 2m extrema. A mode of a so called higher order is one, which has an azimuthal order greater than zero. Thus, a higher azimuthal order is one with an index I>0.
A mode of an azimuthal order equal to zero has a mode pattern that is rotationally invariant. This may be either the fundamental mode, for which the radial order is equal to one, or a higher mode, for which the radial order is higher than one.
For a mode of an azimuthal order equal to or greater than one and a given radial order, there exists also another mode of the same azimuthal order and the same radial order, for which the mode patterns of the two modes are orthogonal to each other, e.g. the surface integral of the mode patterns' product is equal to zero.
The technique of Mode Division Multiplexing (MDM) may be employed within a MMF, wherein different optical signals as different TEM modes of the same wavelength may transmit respective individual data within the MMF.